1. Field of the Invention
The present invention relates to an improved defining member for defining the thickness of a layer of a one-component developer employed for developing an electrostatic latent image, and a developing device equipped with said defining member.
2. Related Background Art
FIG. 6 is a schematic view of a known developing device, wherein a developing sleeve 21, composed of a non-magnetic pipe made for example, of aluminum or stainless steel and serving as a developer bearing member, is provided in a developing container 23 containing a magentic toner 24 constituting one-component developer, at an aperture opposed to an electrophotographic photosensitive member (image bearing member, not shown). Inside said developing sleeve 21, there is unmovably fixed a magnet 22 having plural magnetic poles N, S in an alternating manner, and the toner 24 in the developing container is supported on the surface of the sleeve 21 rotating in the direction indicated by an arrow, by the magnetic function of said magnet 22.
An elastic blade 26, serving as a developer defining member, is mounted on a supporting plate 27, and extends in the opposite direction to the rotating direction of the developing sleeve 21 and elastically contacts, at a lateral face of the free end, the surface of the developing sleeve 21, whereby a nip 12 is formed between the blade 26 and the sleeve 21.
The blade (defining member) 26 directly contacts the sleeve 21 in the absence of the developer, but contacts the developer, instead of the sleeve 21, in case the developer is present on the sleeve 21. In the latter case, however, the blade 26 elastically presses the thin layer of developer to the sleeve 21. Consequently, in the present text, the blade (defining member) is represented as being in contact with the sleeve (developer supporting member).
The elastic blade 26 is in general composed of urethane rubber, silicone rubber or butyl rubber. The toner 24 supported on the developing sleeve 21 is defined in thickness, in passing said nip, whereby a thin toner layer 25 is formed on the developing sleeve 21.
The toner layer 25 supported on said developing sleeve 21 is transported, by the rotation of the developing sleeve 21, to a developing area where the sleeve 21 is positioned close to the image bearing member, and the toner 24 is supplied and deposited on the latent image formed on said image bearing member. Thus, the development of said latent image is achieved.
The lower part of the toner layer 25 on said developing sleeve 21 is sufficiently friction charged to a polarity required for the development of the latent image, by the contact with the developing sleeve 21. On the other hand, the upper part of the toner layer 25 is given a charge by friction charging in contact with the elastic blade 26, but sufficient frictional charge is often not obtained by such blade 26 composed, for example, of urethane rubber. For this reason, the amount of frictional charge in the toner 24 may be significantly different between the upper and lower parts of the toner layer 25, and such charge distribution often leads to drawbacks such as the ghost phenomenon in the developed image or a low image density due to the deficient charge amount in the upper part of the toner image 25.
Such drawbacks tend to appear in the initial stage of operation of the developing device or immediately after the replenishment of toner 24 into the developing container 23, when the frequency of contacts of toner 24 with the developing sleeve 21 is still limited. Such drawbacks are also encountered when the particle size of the toner 24 is small, because this increases the proportion of toner which is transported to the developing area without sufficient contact with the developing sleeve 21.
To avoid these drawbacks, there has been proposed a method of increasing the contact pressure of the elastic blade 26 with the developing sleeve 21, thereby pressing the toner 24 more strongly to the surface of said sleeve 21 , but the toner 24 tends to be adhered in the nip 12 between the blade 26 and the developing sleeve 21, thereby generating streak-shaped unevenness in the toner layer 25.
Also as an alternative method, Japanese Patent Laid-open No. 62-24282 proposes a method of adding a charge control substance in the elastic blade 26. However, if the content of such charge control substance is low in the blade 26, there cannot be obtained the anticipated effect because the charge control substance does not appear on the surface of the elastic blade 26. Though such effect can be obtained by an increase in said content, the elastic blade 26 becomes incapable of exhibiting sufficient elasticity, thus becoming unable to provide a uniform and sufficient contact pressure on the developing sleeve 21.
Furthermore, Japanese Patent Laid-open No. 60-45272 proposed a method of coating a charge control substance on the surface of the elastic blade 26, in order to effect sufficient frictional charging on the toner 24 without losing the rubber elasticity of the elastic blade 26, thereby providing the toner 24 with a sufficient charge. In this proposal, in consideration of the triboelectric series with the toner 24, the charge control substance is selected for example from styrene resin, acrylic resin, polyester resin, epoxy resin, polyamide resin, fluorinated resin, silicone resin or polycarbonate resin.
However, if the charge control substance coated on the elastic blade 26 is poor in lubricating property, said charge control substance may be peeled off by the sliding contact with the developing sleeve 21, particularly in the initial stage of use when the elastic blade 26 is in direct contact with the developing sleeve 21. Also, the poor lubricating character of such charge control substance hinders smooth transportation of the toner 24 by the developing sleeve 21, whereby the toner staying at the elastic blade 26 is excessively charged, thus resulting in a loss in the density, or in speckles on the obtained image due to the coagulation of toner on the developing sleeve 21.
For this reason, Japanese Patent Laid-open No. 63-221369 proposes a method of forming the elastic blade itself with synthetic resin other than rubber and mixing particles of different resin in the resin constituting the blade 26, thereby stabilizing the coating of the toner 24.
However, in order to define the thickness of the toner layer for a prolonged period and to provide the toner 24 with a charge, a considerably high contact pressure to the sleeve has to be given to the elastic blade 26, and the resin blade 26 containing particles of different resin in the entire blade cannot exhibit sufficient elasticity, thus being unable to maintain a uniform and sufficient contact pressure.